Vehicle anti-theft systems commonly include several sensors on the vehicle which are coupled to an alarm controller to determine when an unauthorized intrusion of the vehicle interior occurs. A logical interpretation of the sensor states determines whether the alarm controller should be armed, disarmed, or should activate an alarm. The armed, disarmed or alarm status of the controller affects the interpretation of the sensor input and whether an alarm is actually sounded.
The most common way of defeating an alarm system is to disconnect the vehicle battery; then no power is available to energize an alarm. Further, when the battery is reconnected, the status of the controller will go to some default value unless some arrangement is made to store the status during the period of power loss. It is desirable that the previous alarm status be resumed when the battery is reconnected and that the fact of the battery reconnection be part of the controller information. In general, it is desirable the alarm be sounded when the battery is reconnected unless the alarm status was disarmed or a vehicle key is operative.
Several approaches to dealing with the battery disconnect problem have been proposed. One proposal is to use a low leakage capacitor to maintain power on the controller during battery power loss. This is effective for only short power loss periods, however. Another approach is to use a backup battery for the controller but that is an expensive solution. A temporary state storage may also be used but this also is limited status is remembered, the separate problem exists of detecting the reconnection of the battery.